1. Field of the Invention
The present invention relates to medical devices and techniques for accurately deriving cardiac and breathing parameters of a subject from extra-thoracic blood flow measurements, in particular, the invention relates to medical devices and techniques for deriving breath rate, breath distention, and pulse distention measurements of a subject from a pulse oximeter system coupled to a small animal.
2. Background Information
As background, one type of non-invasive physiologic sensor is a pulse monitor, also called a photoplethysmograph, which typically incorporates an incandescent lamp or light emitting diode (LED) to trans-illuminate an area of the subject, e.g. an appendage, which contains a sufficient amount of blood. The light from the light source disperses throughout the appendage {which is broken down into non-arterial blood components, non-pulsatile arterial blood, and pulsatile blood}. A light detector, such as a photodiode, is placed on the opposite side of the appendage to record the received light. Due to the absorption of light by the appendage's tissues and blood, the intensity of light received by the photodiode is less than the intensity of light transmitted by the light source (e.g., LED). Of the light that is received, only a small portion (that effected by pulsatile arterial blood), usually only about two percent of the light received, behaves in a pulsatile fashion. The beating heart of the subject, and the breathing of the subject as discussed below, create this pulsatile behavior. The “pulsatile portion light” is the signal of interest, and effectively forms the photoplethysmograph. The absorption described above can be conceptualized as AC and DC components. The arterial vessels change in size with the beating of the heart and the breathing of the patient. The change in arterial vessel size causes the path length of light to change from dmin to dmax. This change in path length produces the AC signal on the photo-detector, which spans the intensity range, IL to IH. The AC Signal is, therefore, also known as the photoplethysmograph.
The absorption of certain wavelengths of light is also related to oxygen saturation levels of the hemoglobin in the blood transfusing the illuminated tissue. In a similar manner to the pulse monitoring, the variation in the light absorption caused by the change in oxygen saturation of the blood allows for the sensors to provide a direct measurement of arterial oxygen saturation, and when used in this context, the devices are known as oximeters. The use of such sensors for both pulse monitoring and oxygenation monitoring is known, and in such typical uses, the devices are often referred to as pulse oximeters. These devices are well known for use in humans and large mammals and are described in U.S. Pat. Nos. 4,621,643; 4,700,708 and 4,830,014, which are incorporated herein by reference.
Current commercial pulse oximeters do not have the capability to accurately measure heart rate of non-compliant subjects, particularly small mammals, or to accurately measure breath rate or other breathing related parameters in small subjects.
It is an object of the present invention to minimize the drawbacks of the existing systems and to provide medical devices and techniques for deriving accurate cardiac and breathing parameters of a subject from extra-thoracic blood flow measurements.
It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents, unless expressly and unequivocally limited to one referent. For the purposes of this specification, unless otherwise indicated, all numbers expressing any parameters used in the specification and claims are to be understood as being modified in all instances by the term “about.” All numerical ranges herein include all numerical values and ranges of all numerical values within the recited numerical ranges.
The various embodiments and examples of the present invention as presented herein are understood to be illustrative of the present invention and not restrictive thereof, and are non-limiting with respect to the scope of the invention.
At least some of the above stated objects are achieved with a method of utilizing a conventional pulse oximeter signal to derive accurate heart and breath rate, particularly as applied to small mammals. At least some of the above stated objects are achieved with a method of utilizing a pulse oximeter signal to derive breath rate. As understood by those of ordinary skill in the art, a pulse oximeter is applied to the subject with a simple externally applied clip. Thus, in addition to getting oxygen saturation and heart rate from a pulse oximeter, the pulse oximeter according to the present invention can derive breath rate.
A measurement of breath rate from a pulse oximeter was first made commercially available in December 2005 by the assignee of the present application, Starr Life Sciences Corp., and is provided in the MouseOx™ device that was particularly designed for use with small mammals, namely rats and mice. In this device, the breath rate is obtained by screening out the frequency band around the heart rate point on the Fast Fourier Transform (known as FFT) that is used to identify the heart rate. The next largest amplitude to the left (or lower frequency) of the heart rate rejection band on the FFT is considered to be the breath rate. The value is then simply averaged then displayed on the screen to the user. Although useful there is room to greatly improve this calculation methodology to assure consistent, accurate results.
Pulse oximeter measurements are very susceptible to motion artifact. The reason for this sensitivity is that measurements for pulse oximetry are made at specific points in the cardiac cycle (systole and diastole). Identification of systole and diastole can only be done by observing the change in light absorption that accompanies the small peripheral blood pulse derived from the stroke volume of a cardiac cycle. One source of noise is breathing effort, which affects the blood volume delivered to the periphery in synchrony with the breathing cycle. If effort is high, the change in peripheral blood volume can be greater with breathing than with cardiac stroke volume.
A more pernicious source of noise however, is caused by motion of the tissue and sensors. Because the signal level of transmitted light is so small relative to the baseline transmitted light, very small motions of the tissue relative to the sensor pads can cause large changes in light transmission as the type and amount of tissue that resides between the LEDs and photodiode changes. If these changes happen in a rhythmic manner, it can appear to the analysis software as a cardiac stroke volume pulse, or at the very least, it can swamp the ability of the software to pick out the heart rate signal. Note that the description of these problems is identical to that which would apply to measuring breath rate.